Thermal cracking of hydrocarbons

ABSTRACT

IN THE THERMAL CRACKING OF HYDROCARBONS THE FORMATION OF COKE DEPOSITS AND CONCOMITANT CARBURIZATION OF IRON CONTAINING REACTION ZONE SURFACES IS RETARDED BY CARRYING OUT THE THERMAL CRACKING OPERATION IN A REACTOR WHEREIN THE WALLS OF THE REACTOR IN CONTACT WITH THE PROCESS FLOW HAVE BEEN COATED WITH A PROTECTIVE OVERLAYER OF A METAL SELECTED FROM THE CLASS CONSISTING OF ALUMINUM AND ALUMINUM ALLOYS MELTING BELOW 1250*C., SAID COATING PREFERABLY HAVING INTERSPERSED THEREIN A CATALYTIC ANTI-COKE DEPOSITION SUBSTANCE MADE UP OF A MIXTURE OF IRON (III) OXIDE, CHROMIUM (III) OXIDE AND POTASSIUM CARBONATE. THE PROTECTIVE OVERLAYER OR COATING EMPLOYED IN THE PROCESS OF THE INVENTION IS APPLIED TO THE WALLS OF THE REACTOR IN PARTICULATE FORM IN ADMIXTURE WITH A SOLDERING FLUX WHICH LIQUEFIES AT TEMPERATURES BELOW THE MELTING POINT OF THE PARTICULATE METAL AND CATALYST MIXTURE; THE REACTOR WALLS ARE MAINTAINED AT A TEMPERATURE AT WHICH THE SOLDERING FLUX IS LIQUID FOR A TIME SUFFICIENT TO DISSOLVE A SUBSTANTIAL PORTION OF THE METAL OXIDE PRESENT ON THE REACTOR WALL SURFACE AND THE REACTOR WALLS ARE THEN HEATED TO A TEMPERATURE SUFFICIENT TO MELT THE PARTICULATE METAL WHICH ON COOLING AND REMOVAL OF THE EXCESS FLUX FORMS TH PROTECTIVE OVERLAYER OF THE INVENTION.

United States Patent US. Cl. 208-48 R 4 Claims ABSTRACT OF THEDISCLOSURE In the thermal cracking of hydrocarbons the formation of cokedeposits and concomitant carburization of iron containing reaction zonesurfaces is retarded by carrying out the thermal cracking operation in areactor wherein the walls of the reactor in contact with the processflow have been coated with a protective overlayer of a metal selectedfrom the class consisting of aluminum and aluminum alloys melting below1250 C., said coating preferably having interspersed therein a catalyticanti-coke deposition substance made up of a mixture of iron (III) oxide,chromium (III) oxide and potassium carbonate. The protective overlayeror coating employed in the process of the invention is applied to thewalls of the reactor in particulate form in admixture with a solderingflux which liquefies at temperatures below the melting point of theparticulate metal and catalyst mixture; the reactor walls are maintainedat a temperature at which the soldering flux is liquid for a timesufficient to dissolve a substantial portion of the metal oxide presenton the reactor wall surface and the reactor walls are then heated to atemperature sufficient to melt the particulate metal which on coolingand removal of the excess flux forms the protective overlayer of theinvention.

BACKGROUND OF THE INVENTION Field of the Invention This inventionrelates to an improved process for thermal cracking of hydrocarbonswherein the cycle time between shutdowns due to coke build up in thereaction zone is substantially extended and the detrimental effect ofcarbon corrosion on reactor lifetimes is substantially reduced. Moreparticularly the invention is directed to a thermal hydrocarbon crackingprocess wherein coke formation and concomitant 'carburization of ironcontaining reaction zone surfaces is retarded by coating the reactionzone surfaces with certain metals and inorganic salts and oxides havingprotective'and/or catalytic activity which form a protective overlayerof superior properties when applied to the reaction zone surface by astepwise process wherein the reaction zone surface is first subject tothe metal oxide dissolving action of a conventional soldering flux priorto laying down of the The formation of coke deposits, sometimes calledsludge or pitch deposits depending on their specific chemical andphysical characteristics, on the reaction zone surfaces of thermalhydrocarbon cracking units has long plagued those engaged in hydrocarbonprocessing and refining. Due to cost factors and the need for structuralstrength in the processing equipment, commercial scale thermal crackingunits are typically constructed from an ironcontaining metal suchasstainless steels or other ironcontaining alloyed metals. With.processing equipment of this metal make up, the problem of cokeformation on thermal cracking of hydrocarbon fractions seems to beespecially critical since ferrous surfaces have a marked tendencytowards coke collection and are susceptible to 3,827,967 Patented Aug.6, 1974 carburization or carbon corrosion due to diffusion of carboninto the ferrous (typically stainless steel) surface at thermal crackingtemperatures. Moreover, since the cracking or reaction zone in mostconventional hydrocarbon cracking units comprise a series of tubularreactors or coils operated in parallel, the formation of coke depositson the reaction zone surface can be quickly lead to inefiicientoperation and shutdown of the cracking unit due to increasing pressuredrop, loss of heat transfer through reactor walls and, ultimately, plugoff of the reactor as the coke deposit builds up.

A number of varied means and methods have been suggested to avoid orminimize the problems associated with coke build up in thermalhydrocarbon cracking processes. Most pertinent of these expedients forhandling the coking problem are those which are directed to preventionof the initial deposition of the coke deposit. Several methods aredescribed wherein various chemical substances such as benzene sulfonicacid derivatives (US. 3,105,810), aqueous alkali metal salts (U.S.2,893,941 and 3,617,478) and even light petroleum distillate incombination with subdivided seed particles of coke itself (U.S.2,717,865) have been added to the process stream to prevent cokedeposition on the reaction zone surfaces. A seemingly larger number ofreferences have been directed to methods or means of treating or coatingthe reaction zone surfaces or reactor walls to prevent deposition. Thus,US. 1,689,363 suggests coating the reactor walls with an iron sulphidelayer by treatment with hydrogen sulphide; US. 2,263,366 teaches thatcoke formation is suppressed by coating the reactor walls with zincoxide using conventional means such as galvanization or spraying withmolten metal followed by oxidation of the coating; and US. 2,063,596 isdirected to coating the reaction apparatus with elemental coatings ofiron, chromium, molybdenum and lead by decomposition of a volatile metalcompounde.g., metal carbonyls-on the reaction zone surface. Morerecently, Belgium Pat. 771,- 594 has proposed forming a protective filmof silicon oxide on the reactor surface by steam treatment of arefractory steel containing less than 1% silicon; Japanese Pat. 47/2,601has suggested the employment of a gold or gold copper alloy coatedreaction zone; and US. 3,536,776 teaches the employment of a reactionchamber constructed of or lined with a metal-ceramic material containingparticles of a catalytically inert, refractory solid material, such asalumina, dispersed in chromium to minimize deposition of carbon onreacton zone surfaces.

Even though the methods described above are numerous and varied, none ofthe methods seem to meet the tests for a completely satisfactory method.This is because each method seems to suffer from one or moredeficiencies in that they either do not inhibit coke formation to asatisfactory degree in practice, or they involve expensive materials ofmanufacture or materials which must be removed from the process flow orlastly, that they involve diflicult and expensive modes of preparation,such as coating with materials melting at very high temperatures, whichdistract from their viability in commercial practice.

SUMMARY THE INVENTION It has now been found that the formation of carbondeposits and the concomitant carburization of iron-containing reactionzone surfaces in contact with the process flow in a thermal hydrocarboncracking zone can be substantially retarded by carrying out a vaporphase, thermal cracking of hydrocarbon feedstocks in a reaction zonewherein the iron-containing reaction zone surfaces in contact with theprocess flow have been coated with an overlayer of aluminum or aluminumalloy melting below about 1250" C. by a process which comprises:

(1) Coating the iron-containing reaction zone surface with particulatealuminum or aluminum alloy melting below about 1250 C. in intimateadmixture with a soldering flux having a melting point below that of theparticulate aluminum or aluminum alloy employed;

(2) Maintaining the coated reaction zone surface at a temperature belowthe melting point of the particulate aluminum or aluminum alloy at whichthe soldering flux is liquid for a period of time sufiicient to allowthe soldering flux to thoroughly cover the reaction zone surface anddissolve any metal oxide present thereon;

(3) Heating of the reaction zone surface covered with said liquid fluxto a temperature sufficient to substantially melt the particulatealuminum or aluminum alloy;

(4) Cooling of the reaction zone surface to form a solid overlayer ofaluminum or aluminum alloy thereon; and

(5) Removing the excess flux from the cooled reaction zone surface.

In a preferred aspect of the process of the invention, which isespecially applicable to thermal cracking of hydrocarbon mixtures suchas naphtha and gas oil in the I presence of steam at temperatures ofabout 400 to about 1000 C. to yield low molecular weight unsaturatedhydrocarbons and gasoline components, a mixture of iron (ferric) (III)oxide, chromium (III) oxide and potassium carbonate is added inparticulate form to the admixture of aluminum or aluminum alloy andsoldering flux employed in step (1) of the above process. This compositemixture of particulate aluminum or aluminum alloy, ferric (III) oxide,chromium (III) oxide and potassium carbonate in admixture with thesoldering flux when subject to the processing steps (1) through (5)described above, wiil yield an aluminum or aluminum alloy coating havinginterspersed therein a ferric (III) oxide, chromium (III) oxide andpotassium carbonate composite mixture which exhibits both protectiveproperties against carburization of the iron-containing reaction zonesurface and superior catalytic activity against coke formation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of the invention inits broad aspects involves an improved process for thermal cracking ofhydrocarbons wherein coke deposition or formation on iron-containingreaction zone surfaces and the concomitant carburization of suchsurfaces in contact with the process flow is substantially retarded. Thebasic thermal hydrocarbon cracking process contemplated by thisinvention is the conventional non-catalytic cracking process which iswell known to those skilled in the art. Though numerous variations andmodifications in terms of equipment design, process flow, and the likeare both within the purview of those skilled in the art and within thescope of this invention, the thermal or non-catalytic cracking processof this invention can be described in general terms as the conversion ofa hydrocarbon feedstock into hydrocarbons having fewer carbon atomsand/or a greater proportion of unsaturated bonds per molecule than thefeedstock composition by passing the hydrocarbon feedstock, which hasbeen optionally preheated, into a reaction zone, optionally in thepresence of injected steam, wherein the hydrocarbon feedstock in thevapor state is subject to elevated temperatures and residence timessufficient to crack or thermally convert it into the desired lowermolecular weight and/or more unsaturated product.

A broad spectrum of petroleum or other naturally occurring hydrocarbonderived feedstocks may be processed in this basic process, or variationsthereof, to yield a variety of hydrocarbon products. Suitable feedstocksinclude light saturated and unsaturated aliphatic hydrocarbons such asethane propane butane, butylene pentane, cyclopentane, butadiene and thelike; the so called intermediate refinery fractions such as naphthas,kerosenes,

' gasolines and gas oils; and the heavier hydrocarbon stocks which canbe vaporized in conventional thermal cracking equipment.

While it will be recognized that the temperature and residence time inthe thermal cracking reactions zone can vary widely with specificfeedstocks composition,desired products and feedstock. conversion; bestyields are generally obtained through employment'of short residencetimes in combination with high reaction zone temperatures. Typically,reaction zone temperatures ranging from about 400 to about 1500 C. incombination with residence times of from about 5 to less than 0.1seconds. A preferred aspect of this invention involves thermal crackingof intermediate hydrocarbon fractions such as mixtures-of naphtha andgas oil in the presence of steam. In this preferred aspect reaction zonetemperatures ranging from about 400 to about l C. are quite suitablewith temperatures in the range of about 700 to 900 C. being preferred.The pressures employed in the cracking process may suitably range from apressure adequate to produce the de-' sired flow through the reactionzone up to slightly elevated pressures of, for example 100 p.s.i. Thehigher,

pressures are typically employed in thermal cracking of gas oils orlighter fractions to produce unsaturated products.

The thermal cracking reaction zone typicaly consists of a series oftubes or coils of diameters ranging between about 5 and 25 cm. which aredisposed in a cracking furnace and connected in parallel to the processflow. Due

to the severity of the conditions employed in the cracking operationthese tubes or coils are typically fabricated out of iron containingalloys such as stainless steel. The substantial benefits derived fromthis invention are based on employment of a reactor wall made up of aniron-containing metal, accordingly, it is contemplated that the thermalcracking zone, irrespective of its specific configuration Will bebounded by reactor walls made up of an iron-containing metal materialsuch as stainless steel.

The substantial benefit of the invention-i.e., substantial retardationof coke formation and concomitant carburization of iron-containingreaction zone surfaces in contact with the process flow is obtained whenthe thermal cracking of the hydrocarbon feedstock is carried out in areaction zone coated on the process side with an overlayer of aluminumor an aluminum alloy melting below about 1250" (3., applied via thecoating process of the invention. This coating process afiords aneconomical means of applying the protective overlayer which is free ofmany of the operational ditficulties associated with the prior artcoatings and coating processes. Moreover, this protective overlayer laiddown by this process is sufiiciently adherent, tough and uniform thatsuperior protective properties are obtained over extended .runtimes inthermal cracking units. .Acordingly, this method of coating theiron-containing reaction zone surfaces in contact with the process flowwith the protective overlayer described forms an integral aspect of theprocess of the invention. Briefly stated, the coating processcontemplated by this invention is a multistep process which comprises:(1) Coating the iron-containing reaction zone surface 'with particulatealuminum or aluminum alloy melting 'below about 1250" C. in intimateadmixture with a soldering flux having a melting point below that of thepar (4) Cooling of the reaction zone surface to form a solid overlayingof aluminum or aluminum alloy thereon; and

(*5) Removing the excess flux from the cooled reaction zone surface.

As indicated above, in the first step of the coating process ofparticulate aluminum or aluminum alloy is employed. The aluminumstarting material suitable for use in the invention is a metallicaluminum of reasonably high purity such as that available in commercefrom a variety of manufacturers. The aluminum alloys which are useful inthe process include all those aluminum alloys which melt below about1250" C. Examples of such alloyed materials suitably include aluminumalloys of one -or more of the following metals: silicon, copper, gold,silver, zinc, tin, lead and iron. Preferably the aluminum componentcomprises a major portion of the alloyed material with alloys or atleast 50% by weight aluminum being prefer red and alloys of 70% or moreby weight aluminum' being most preferred. While the maximum meltingtemperatures of the alloyed materials should not exceed about 1250 C.,the melting point can vary over a wide range below that limit.Typically, alloys containing the preferred proportions of aluminum meltbetween about 600 and about 1000 C., accordingly melting points in thisrange are preferred. The particle size of the particulate aluminum oraluminum alloy may also vary within wide limits, with the only reallimitation being a practical one as to the maximum particle size whichcan be informly applied as an adherent particulate coating by thismethod. Typically, the aluminum or aluminum alloy starting materials areapplied as powders to the reaction zone surface. Such powders suitablyhave a mean particle diameter of from 35 to400 mesh.

The term soldering flux comprises the commercially available mixtureswhich contain a number of chemical compounds which in the liquid stateare capable of dissolving metal oxides present as a skin or impurity onthe surface of metals. Examples of such chemical compounds include zincchloride, ammonium chloride, and the socalled brazing fluxes such asalkali metal borates with or without boric acid, alkali metalbifluorides and other halides. Preference is given to the use of asoldering flux which has a melting point in the range of 70 to 100 C.Examples of such preferred fluxes include the organic fluxes such asrosin.

' The ratio 'of tlie quantity of soldering flux to the quantity ofparticulate aluminum or aluminum alloy can also be selected over a-widerange Quantities by weight of soldering flux in-excess of the-quantityof weight of particulate aluminum or aluminum'alloy generally showadvantages; aweight ratioof soldering flux to metal powder of from 11 Mto lOr'l'is very suitable;

In order to achieve good adhesion of the iron-containing reaction zonesurface and the admixture described in the first step of the process, itis preferred to activate the reaction zone surface prior to theapplication of the coating preferably by abrasion and/ or pickling.Abrasion may be carried out by. sand-blasting or grit-blasting. Once thereaction zone surface has been activated, the admix ture of soldering,flux and the desired particulate metal composition can be applied to theactivated surface by any conventional means such'as spraying, painting,shaking and the like. p p

Second step on the process maytake place at any temperature at which thesoldering. flux flows but at which the metal powder does not melt. If asolid soldering flux is employed it is advisable to select a temperaturewhich is only slightly (about 50 C.) in excess of the flow point of theselected soldering'flux in order to avoid excessive oxidation'of thecarrier metal and the metal powder. If use is made of aliquidsoldering-flux, this heating may be avoided. The soldering flux ismaintained in liquid or fluid state in this step of the process for atime suflicient to dissolve all or substantially all of the metal oxidewhich is present on the reaction zone surface. Time periods ranging fromless than one minute up to about 30 minutes are generally quitesufficient to accomplish this result.

After the carrier metal coated with the mixture of soldering flux andparticulate aluminum or aluminum alloy as described under (1) has beenmaintained at a temperature at which the soldering flux is liquid butbelow the melting point of the particulate metal for a short while asdescribed under (2), the reaction zone surface is further heated asspecified under (3), preferably to a temperature which is at most 300 C.above the melting point of the particulate metal, this temperaturesubsequently being maintained for 1-5 minutes. During this treatment theparticulate aluminum or aluminum alloy adheres to the reaction zonesurface and forms a coating which imparts the desired properties to theiron-containing reaction zone surface.

After heating to melt the metal as specified above, cooling takes place,generally by exposure to the atmosphere. After cooling, excess solderingflux, which is of no further importance in the use of the carrier metalwith the applied coating may be wholly or partially removed, if desired,for example by washing ofi, preferably with warm water.

While the coating process employed in this invention can be utilized tocoat the process side of an iron-containing reaction zone surface of anyconceivable configuration which might be employed in thermal cracking ofhydrocarbons, the process is particularly suitable for coating theinternal walls of tubular reactors which are most widely employed inthis process. In this preferred aspect, during the application of thecoating (steps (1), (2) and (3) of the process described above) the tubeis kept rotating around its longitudinal axis. A number of revolutionsper minute resulting in a rotational speed of the tube wall of 0.5-5m./sec. is very suitable. After the admixture of soldering flux andparticulate aluminum or aluminum alloy has coated the inside of the tubeunder the temperature conditions specified under (2), the temperature israised to the values specified under (3). This need not take placesimultaneously over the entire length of the tube, but may very suitablybe achieved by moving a heat source (for example a concentric ringburner or an induction heat source) longitudinally along the exterior ofthe tube. This movement can effectively be carried out at a speed of afew meters per hour (e.g., 1-10 m./hr.). This speed is naturallydependent on such factors as the heat production of the heat source, thedesired temperature, the wall thickness and the diameter 02 the tube.The tube diameter is usually between 5 and 2 cm.

As indicated above, a preferred aspect of this invention involves thesubstantial retardation of coke formation and concomitant carburizationof iron-containing reaction zone surfaces which occurs on thermalcracking of intermediate hydrocarbon fractions such as naphtha and gasoil, or mixtures thereof, in the presence of steam at temperatures offrom about 400 to about 1100 C. to yield lower molecular weightunsaturated hydrocarbons such as ethylene and propylene, and gasolinecomponents such as isopentane and 2,3-"climethylbutane. In thisapplication it has been found that 'an'aluminum or'aluminum" alloycoating applied to the iron-containing reaction zone surfaces in contactwith the process flow according to the coating process of the inventiongives superior protective results whena composite mixture of ferric(III) oxide,'chromium (III) oxide and potassium carbonate isinterspersed in the protective coating. These three'compone'nts (Fe O CrOg and K CO in combination seem to exhibit superior catalytic anti-cokedeposition properties in this thermal cracking process. Accordingly, inthis preferred aspect, a mixture of ferric (lIDoxide, chromium ('I'II)oxide and potassium carbonate, in particulate form either as individualcomponents or as an intimate admixture, is added to the admixture ofsoldering flux and aluminum. or aluminum alloy employed in the firststep of the coating process of the invention described above. Theremaining steps of the coating process (steps (2)(5) above) are thencarried out substantially as described to yield this superior protectivecoating. The relative proportions of the 3 catalytic anti-cokedeposition components based on the sum of the three components may varywidely. Mixtures consisting of 50-70 parts by weight ferric (III) oxide,25-40 parts by weight potassium carbonate and 1-5 parts by Weightchromium (III) oxide are quite suitable with mixtures containing 60-70parts ferric (III) oxide, 30-35 parts potassium carbonate and 13 partschromium (III) oxide (all by weight) being preferred. This mixture ofcatalytic anti-coke deposition components is preferably used in aquantity of from to 100% by weight (based on the sum of the weight ofthe three components) of the total quantity of soldering flux andparticulate aluminum or aluminum alloy employed. It is most preferred toemploy the 3 catalytic anti-coke deposition components on an approximate equal weight basis with the aluminum or aluminum alloy employed(based on the sum of the weight of three components) ILLUSTRATIVEEMBODIMENT I A mixture of 0.75 g. aluminum and 3.75 g. soldering flux inpowder form was introduced into a test tube of heat-resistant Cr-Nisteel (25% Cr, 20% Ni) having a length of 80 cm. and a diameter of 1 cm.The tube had been given a thorough grit-blasting pretreatment to cleanand activate the surface thereof.

The tube was then heated to a temperature of 80-100 C. under rotationwith a circumferential velocity between 0.5 and 1 m./sec. Thus an evenlayer of flux and coating components was formed. Next the aluminumbinder material was made to adhere firmly to the Wall of the tube by amelting/diffusion process at 850 C. This was effected by moving a heatsource from one end of the tube to the other end with a speed of 24m./hr.

The tube subsequently was allowed to cool and excess flux was removed bywashing with water.

In order to demonstrate the protecting effect of the coating a mixtureof 345 m1./hr. of gas/ oil (density 0.82) and 285 ml./hr. of water wasfed to the coated tube. The mixture was preheated at 500 C. to achievecomplete evaporation, the residence time in the tube was 0.3 sec. basedon total mixture in the hot zone, the pressure was 1.7 atm. gauge andthe exhaust temperature was 830 C. The product mixture was cooled inconventional equipment. After a run period of 118 hours blockage bydeposits was observed in the equipment downstream of the coated tube.The time lapsed before also considerable amounts of carbon weredeposited on the internal wall of the tube was 240 hours.

For comparison an uncoated tube was tested under the same conditions. Inthis case already after 65 hours the operation had to be discontinuedbecause of blockage in the tube.

Another tube was tested after having been aluminum coated in aconventional manner, i.e., by filling the tube with a mixture of blendedaluminum powders, and A1 0 followed by heating to a temperature of 800C.

After only 39 hours the run had to be discontinued due to severe carbonformation.

ILLUSTRATIVE EMBODIMENT Hi ILLUSTIIATIVE EMBODIMENT m Similarly a testwas carried out with a tube internally coated as described in Example I,starting from a mixture of 3.75 g. soldering flux, 1.5 g.aluminum-silver-silicon alloy (16% Ag, 4% Si) and 1.5 g. of a mixture of64.4 parts by weight of ferric (III) oxide, 2.0 parts by weight ofchromium (III) oxide and 33.6 parts by weight of potassium carbonate,the sum being parts by weight.

The run time before clogging of the tube occurred was hours.

What is claimed is: v

1. An improved process for vapor phase, thermal cracking of hydrocarbonfeedstocks wherein the formation of carbon deposits and concomitantcarburization of iron-containing reaction zone surfaces in contact withthe process flow is substantially retarded by the improvement whichcomprises carrying out said thermal cracking process in a reaction zonewherein the iron-containing reaction zone surfaces in contact with theprocessflow have been coated with an overlayer of aluminum or aluminumalloy melting below about 1250 C. by the process of:

(l) coating the iron-containing reaction zone surface with particulatealuminum or aluminum alloy melting below about 1250 C. in intimateadmixture with a soldering flux having a melting point below that of theparticulate aluminum or aluminum alloy employed;

(2) maintaining the coated reaction zone surface at a temperature belowthe melting point of the particulate aluminum or aluminum alloy at whichthe soldering flux is liquid for a period of time sufficient to allowthe soldering flux to thoroughly cover the reaction zone surface anddissolve any metal oxide present thereon;

(3) heating the reaction zone surface covered with said liquid flux to atemperature sufficient to substantially melt the particulate aluminum oraluminum alloy;

(4) cooling of the reaction zone surface to form a solid overlayer ofaluminum or aluminum alloy thereon; and p (5) removing the excess fluxfrom the cooled reaction zone surface.

2. The process according to claim 1 wherein the hy drocarbon feedstockis gas, oil or naphtha, or mixtures thereof, and the thermal cracking iscarried out at temperatures of about 400 to 1000 C. in the presence ofsteam.

3. The process according to claim 2 wherein a mixture of ferric (III)chloride chromium (III) oxide and potassium carbonate is added inparticulate form to the admixture of aluminum or aluminum alloy andsoldering flux employed in step (1) of the coating process and theremaining steps of said coating process are carried out to yield analuminum or aluminum alloy coating having interspersed therein a ferric(III) oxide, chromium (III) oxide and potassium carbonate compositemixture.

4. The process of claim 3 wherein the mixture of ferric (III) oxide,chromium (III) oxide and potassium carbonate added to the admixture ofaluminum or aluminum alloy and soldering flux in step (1) contains 50-70parts by weight ferric (III) oxide, 1-5 parts by weight chromium (III)oxide and 25-40 parts by weight potassium carbonate based on the totalweight of the 3 component mixture added.

References Cited UNITED STATES PATENTS 1,689,363 10/1928 Perl 208-48 R2,063,596 12/1936 Feiler 20848 R 2,263,366 11/1941 Teclc et al. 20848 R3,536,776 10/1970 Lo 260--683 R DELBERT E. GANTZ, Primary Examiner G. E.SCHMITRONS, Assistant Examiner US. Cl. X.R.

23-252 A; 208-l06; 260-4583 R

